Tracking control of a digital audio tape recorder (DAT) is made by an automatic tracking following system (ATF system), the principles of which will be described with reference to FIGS. 2 through 4.
FIG. 4A illustrates the relationship between a rotary drum 40 of a DAT and a running path of a magnetic tape 41. The rotary drum 40 has two heads ha and hb (not shown) provided at the same height but spaced apart from each other on the periphery of the drum so that the same movement locus can be obtained. The rotary drum 40 rotates in a direction indicated by J at 2000 r.p.m. The magnetic tape 41 moves in a direction indicated by K at a tape velocity of v along the tape running path. The magnetic tape is provided with guide rollers 44 and 45 for controlling its movement in widthwise direction along the rotary drum 40 and inclined pins 42 and 43. An angle at which the magnetic tape 41 is wound on the rotary drum 40 is set to be approximately 90 degrees if an outer diameter of the rotary drum 40 is 30 mm, while an angle between the tape running direction and a rotary axis of the rotary drum 40 is equal to a reference angle .theta.r.
FIG. 4B illustrates the reference angle .theta.r between the tape running direction at the tape winding portion and the rotary axis of the rotary drum 40 in a plane face.
Referring now to FIG. 2, there are shown the ATF areas of track pattern in an enlarged manner. The ATF areas are indicated by E.sub.1 and E.sub.2 adjacent to the edges of the magnetic tape 41 in a widthwise direction and a PCM area is indicated by E.sub.3 on which a PCM signal is recorded. The ATF areas are formed of blocks P (indicated by hatching) on which pilot signals s.sub.1 are recorded, respective blocks A and B (indicated by lateral lines and longitudinal lines, respectively) on which synchronizing signals s.sub.2 of different frequencies corresponding to the heads ha and hb, respectively, are recorded and blocks D on which IBG signals are recorded.
On reproduction of the signals, the heads ha and hb, which have a width equal to 1.5 times the track width, alternately scan the adjacent tracks of the magnetic tape to reproduce the signals on the magnetic tape. FIGS. 3A, 3B and 3C show the pilot signals s.sub.1 reproduced when the head hb scans the track t.sub.4. The head hb sequentially scans the blocks P.sub.51, P.sub.31 and P.sub.41 of the ATF area E.sub.1 and the blocks P.sub.42, P.sub.52 and P.sub.32 of the ATF are E.sub.2 to reproduce the pilot signals s.sub.1 recorded on these blocks.
FIG. 3A shows waveforms of the reproduction level of the pilot signals s.sub.1 when the head hb normally scans the track T.sub.4. As noted from FIG. 3A, at the ATF area E.sub.1 the reproduced pilot signals s.sub.1 on the block P.sub.31 on the leftward adjacent track T.sub.3 and the block P.sub.51 on the rightward adjacent track T.sub.5 have the same level. Also, at the ATF area E.sub.2, the reproduced signals on the blocks P.sub.32 and P.sub.52 have the same level. This means that at the ATF areas E.sub.1 and E.sub.2 the width at which the head hb scans the track T.sub.3 is equal to the width at which it scans the track T.sub.5 while a center of the width of the track T.sub.4 aligns with a center of the width of the head hb.
FIG. 3B shows waveforms of the reproduced signals when the head hb scans the track T.sub.4 in an offset manner toward the rightward track T.sub.5. It will be noted that the width at which the head hb scans the rightward adjacent track T.sub.5 increases while the width at which the head hb scans the leftward adjacent track T.sub.3 decreases. Accordingly, as noted from FIG. 3B, the reproduction level of the pilot signal s.sub.1 recorded on the track T.sub.5 increases in proportion to the offset of the head hb while the reproduction level of the pilot signal s.sub.1 recorded on the track T.sub.3 decreases in proportion thereto.
FIG. 3C shows waveforms of the reproduction levels of the pilot signals s.sub.1 when the head hb scans the track T.sub.3 in an offset manner toward the leftward adjacent track T.sub.3. At that time, the reproduction level of the pilot signal s.sub.1 on the track T.sub.3 increases while that on the track T.sub.5 decreases.
In this manner, it will be noted that if the head scans in an offset manner from the predetermined track, then the reproduction of the pilot signal s.sub.1 on the adjacent track increases or decreases in accordance with the direction and magnitude of offset.
ATF control is accomplished by sequentially detecting a difference between the pilot signals s.sub.1 reproduced on the blocks on the adjacent tracks and controlling a rotational velocity of a capstan, which determines the velocity v of the magnetic tape, so that the reproduction levels are equal to each other.
FIG. 7 shows a conventional ATF control system which accomplishes the aforementioned ATF control.
A reproduction signal s.sub.0 amplified by an RF amplifier 1 is applied to a low-pass filter 2 for detecting the pilot signals s.sub.1 and also to a band-pass filter 3 for detecting the synchronizing signal s.sub.2. An envelope detection circuit 4 receives the pilot signal s.sub.1, having a reproduction frequency of 130.67 kHz, and supplies a level voltage signal s.sub.3, which corresponding to the level of the pilot signal s.sub.1, to a subtracter 6 and also to a sample and hold circuit 5 (hereafter referred to as SH circuit). A control circuit 34 receives the synchronizing signal s.sub.2 and supplies pulse control signals s.sub.6 and s.sub.7 at a predetermined timing when the frequency of the synchronizing signal s.sub.2 corresponding to the scanning head is confirmed. For example, while the head hb scans the track T.sub.4 in FIG. 2, it detects the synchronizing signal s.sub.2 when it reaches the block B.sub.41. When the control circuit 34 confirms that the frequency of the synchronizing signal s.sub.2 corresponds to the head hb, it outputs the control signal s.sub.6 at the timing t.sub.1 of FIG. 3 and the control signal s.sub.7 at the timing t.sub.2 of FIG. 3, respectively. The SH circuits 5, 32 hold input signals by their control signals s.sub.6, s.sub.7, respectively. Thus, it will be noted that the SH circuit 5 holds the level voltage signal of the pilot signal detected from the block P.sub.51 on the rightward adjacent track T.sub.5 while the SH circuit 32 holds a level difference signal s.sub.4 corresponding to the difference between the level voltage signal held by the SH circuit 5 and the level voltage of the pilot signal detected from the block P.sub.31 on the leftward adjacent track T.sub.3. The holding operations are made every time the respective heads ha and hb detect the corresponding synchronizing signals s.sub.2 and, therefore, the hold signal output from the SH circuit 32 becomes a tracking error voltage signal s.sub.5 (hereafter referred to as TE voltage signal) indicating a tracking error.
The TE voltage signal s.sub.5 is smoothed by a smoothing circuit 33 to provide an average voltage signal s.sub.12, which is input to a drive circuit 12 for a capstan driving motor 13 which forms a tape transport means. The capstan motor drive circuit 12 rotationally drives a capstan motor 13 so that the input average voltage signal s.sub.12 becomes 0 level and the tape velocity v is controlled by negative feedback on the average voltage signal s.sub.12 by means of the ATF control system.
FIGS. 5A and 5B show variations in loci of the synchronizing signal s.sub.2 ' and the level voltage signal s.sub.3 ' detected corresponding to the head hb when the center position of the head hb scanning the ATF area E.sub.1 moves over the respective tracks. It should be noted that the synchronizing signal s.sub.2 ' and the level voltage signal s.sub.3 ' are different from the synchronizing signal s.sub.2 and the level voltage signal s.sub.3.
A variation in level of the synchronizing signal s.sub.2 ' shown in FIG. 5A will be described with reference to FIG. 2. A level Vs.sub.4 of the synchronizing signal s.sub.2 ' detected on the block B.sub.41 becomes the maximum level when the center of the head hb scans the center of the track T.sub.4. The level Vs.sub.4 never varies until the head hb is offset by a quarter of the track in the positive direction toward the rightward adjacent track T.sub.5. However, it linearly decreases when the head hb is further offset in the positive direction because the width at which the head hb passes through the block B.sub.41 decreases and becomes zero level when the head hb reaches a position d.sub.4. At position d.sub.4 the head hb is offset by a quarter of the track width from the center of the track T.sub.5. On the other hand, a level Vs.sub.6 of the synchronizing signal s.sub.2 ' detected on the block B.sub.61 of the track T.sub.6 linearly increases when the head hb passes over a position d.sub.3 and becomes the maximum level at a position d.sub.6. Thus, the variation in level is repeated in the same manner as the level Vs.sub.4 in accordance with the movement of the head hb in the positive direction.
It will be apparent that when the head hb is offset in the negative direction toward the leftward adjacent track T.sub.3, the synchronizing signal s.sub.2 ' is also detected and varies as shown in FIG. 5A. In FIG. 5A, a level Vs.sub.2 is a level of the synchronizing signal s.sub.2 ' detected on the block B.sub.21 on the track T.sub.2.
A variation in level of a level voltage signal s.sub.3 ' obtained in accordance with the movement of the head hb will hereafter be described with reference to FIGS. 5B and 2.
After it detects the synchronizing signal s.sub.2 ' having a higher level than a critical value Vf on the blocks B, the control circuit 34 outputs control signals s.sub.6 and s.sub.7 to detect levels of the pilot signals on the adjacent tracks at the timings of t.sub.1 and t.sub.2 of FIG. 3A-3C. Thus, when the head hb lies within the range of w.sub.2 and the level of the level voltage signal s.sub.3 ' is on the blocks P, the head hb scans at the predetermined timing of t.sub.1 and t.sub.2 after it passes through the blocks B.sub.21 where the synchronizing signal is detected. When the head hb lies within the range of w.sub.4, and the level of the level voltage signal s.sub.3 ' is on the blocks P, the head hb scans at the predetermined timing of t.sub.1 and t.sub.2 after it passes through the blocks B.sub.41 where the synchronizing signal is detected. When the head hb lies within the range of w.sub.6, and the level of the level voltage signal s.sub.3 ' is on the blocks P, the head hb scans at the predetermined timing of t.sub.1 and t.sub.2 after it passes through the blocks B.sub.61 where the synchronizing signal is detected.
Once the control circuit 34 detects the synchronizing signal, the subsequent synchronizing signal is not detected until the control signals s.sub.6 and s.sub.7 on the synchronizing signal are output. Therefore, the synchronizing signal at the dotted line portions of FIG. 5A is never detected.
Accordingly, when the scanning center of the head hb lies at the center of the track T.sub.4, a level Vp.sub.1 of the level voltage signal s.sub.3 ' provided at the timing of t.sub.1 and indicated by a solid line and a level Vp.sub.2 of the level voltage signal s.sub.3 ' provided at the timing of t.sub.2 and indicated by a dotted line correspond to a detected level of the pilot signal detected by the head hb on the blocks P.sub.51 and P.sub.31, respectively, and have the same level as each other as shown in FIG. 5B. When the scanning position of the head hb is offset in the positive direction, the level Vp.sub.1 linearly increases and the level Vp.sub.2 linearly decreases. When the head hb reaches the position d.sub.1, the level Vp.sub.2 becomes zero and, when it reaches the position d.sub.3, the level Vp.sub.1 becomes maximum. Furthermore, when the head hb passes over the position d.sub.3, it begins to detect the pilot signal on the block P.sub. 61 at the timing of t.sub.2 and, therefore, the level Vp.sub.2 linearly increases. When the head hb passes over the position d.sub.7, where the detected level of the synchronizing signal on the block B.sub.61 exceeds the critical value Vf, it falls within the range w.sub.6 and the level of the level voltage signal s.sub.3 ' on the block P at the predetermined timing of t.sub.1 and t.sub.2 is provided after it passes over the block B.sub.61. Accordingly, the level Vp.sub.1 becomes zero after it reaches the position d.sub.6 and remains zero until it reaches the position d.sub.7 where the pilot signal on the block P.sub.71 is detected. On the other hand, the level Vp.sub.2 becomes the detected level of the pilot signal on the block P.sub.51, is kept at the maximum level until it reaches the position d.sub.4 and linearly decreases when it passes thereover. Finally, when the scanning center of the head hb reaches the center of the track T.sub.6, the levels Vp.sub.1 and Vp.sub.2 again are equal, but the respective levels become the detected levels of the pilot signals detected by the head hb on the blocks P.sub.51 and P.sub.71, respectively. Also, when the head hb is offset in the negative direction, the levels Vp.sub.1 and Vp.sub.2 of the pilot signals s.sub.3 ' are similarly detected and vary as shown in FIG. 5B.
A variation in locus of the level voltage signal s.sub.3 ' is detected when the scanning position of the head hb moves over the respective tracks at the ATF area E.sub.2 will hereafter be described with reference to FIGS. 5A, 5C and 2.
At that time, variation in the locus of the synchronizing signal s.sub.2 ', varying in accordance with the scanning position of the head hb will be identical to that at the ATF area E.sub.1. However, variation in the locus of the level voltage signal s.sub.3 ' is slightly different from the aforementioned one, as shown in FIG. 5C. More particularly, when the scanning center of the head hb is offset from the center of the track T.sub.4 in the positive direction, the respective levels Vp.sub.1 and Vp.sub.2 vary in the same manner as in the ATF area E.sub.1 to the position d.sub.3. However, when it passes through the position d.sub.3, there is no block P detected at the timing of t.sub.2 and therefore the level of Vp.sub.2 never increases as at the ATF area E.sub.1 and, therefore, is kept at zero. On the other hand, the level Vp.sub.1 becomes the detected level of the pilot signal on the block P.sub.42 after the head hb reaches the position d.sub.7 where the detecting blocks of the synchronizing signal changes from B.sub.42 to B.sub.62 and linearly decreases as the head hb moves in the positive direction, as shown in FIG. 5C.
Although only the variation in locus of the level of the pilot signal detected at the ATF areas E.sub.1 and E.sub.2 when the scanning position of the head hb moves between the tracks, the above discussion is also true of the variation in the level of the pilot signal detected when the head ha moves between the tracks. However, it should be noted that in this case, the scanning position of the head ha and the variation in locus of the pilot signal level is offset by one track as indicated by (T.sub.2), (T.sub.3) ------ and (T.sub.8) in FIGS. 5A through 5C. The variation in locus of the pilot signal level at the ATF area E.sub.1 is shown by FIG. 5C while that at the ATF area E.sub.2 is shown by FIG. 5B. As noted from this, they are opposite of those of the head hb. This is due to the fact that the position of the block B having the synchronizing signal corresponding to the head hb and the block P having the pilot signal at the ATF area E.sub.1 is consistent with the relation of position of the block A having the synchronizing signal corresponding to the head ha and the block P having the pilot signal at the ATF area E.sub.2.
The TE voltage signal s.sub.5 from the SH circuit 32 of FIG. 7 corresponds to a differential voltage (Vp.sub.1 -Vp.sub.2) between the levels Vp.sub.1 and Vp.sub.2 obtained in synchronization with the synchronizing signal meeting the aforementioned conditions. FIG. 5D shows a variation in level which is obtained by subtracting the dotted line from the solid line of FIG. 5B while FIG. 5E shows a variation in level which is obtained by subtracting the dotted line from the solid line of FIG. 5C. Thus, it will be understood that a differential voltage Va.sub.1 is obtained by detecting the synchronizing signal corresponding to the ATF area E.sub.1 where the head ha is positioned while the scanning position moves between the tracks. Likewise, a differential voltage Vb.sub.2 is obtained by detecting the synchronizing signal corresponding to the ATF area E.sub.2 where the head hb is positioned while the scanning position moves between the tracks vary, as shown in FIG. 5E. It will also be understood that a differential voltage Vb.sub.1 is obtained by detecting the synchronizing signal corresponding to the ATF area E.sub. 1 where the head hb is positioned while the scanning position moves between the tracks and a differential voltage Va.sub.2 is obtained by detecting the synchronizing signal corresponding to the ATF area E.sub.2 where the head ha is positioned while the scanning position moves between the tracks vary, as shown in FIG. 5D.
Now, supposing that the rotary drum having heads ha and hb disposed at identical heights on recording and moving along the identical locus rotates at 2000 r.p.m. and having an angle between the axis of the rotary drum and the tape running direction being equal to the reference angle .theta.r which corresponds to the angle on recording and that the magnetic tape runs at the tape velocity Vp which is approximately equal to the tape velocity on recording for reproducing the ATF signal from the magnetic tape, the scanning positions of the heads ha and hb scanning the ATF area E.sub.1 and E.sub.2 are identical to each other relative to the center of the corresponding tracks. In this case, the differential voltages Vb.sub.1 and Va.sub.2 on the variation of FIG. 5D and the differential voltages Va.sub.1 and Vb.sub.2 on the variation of FIG. 5E move along the identical axis.
Supposing that the heads ha and hb are offset in height, the scanning positions of the heads relative to the center of the corresponding to the tracks Tr at the ATF area E.sub.1 have a height error wh corresponding to the offset height. This is also true of the relation of their positions at the ATF area E.sub.2. These height errors wh can be expressed as errors between the movement positions of the differential voltages Va.sub.1 and Vb.sub.1 and between those of the differential voltages Va.sub.2 and Vb.sub.2 in FIGS. 5D and 5E, respectively. These figures correspond to the case in which the head hb is positioned lower than the head ha and the lower head is offset in the negative direction.
Supposing that the angle between the axis of the rotary drum and the tape running direction is inclined relative to the reference angle .theta.r on recording, the scanning direction of the heads is never parallel to the direction of the tracks on recording. Thus, the scanning positions of the head ha relative to the center of the corresponding track at the ATF area E.sub.1 and relative to the center of the corresponding track at the ATF area E.sub.2 have an inclination error ws in accordance with their inclination. This is also true of the head hb. These inclination errors ws can be expressed as errors between the movement positions of the differential voltages Va.sub.1 and Vb.sub.1 and between those of the differential voltages Va.sub.2 and Vb.sub.2 in FIGS. 5D and 5E, respectively. This corresponds to the case in which the angle between the axis of the rotary drum and the tape running direction is offset by -.DELTA..theta. relative to the reference angle .theta.r in the angle relation of FIG. 4B. In this case, the relation of FIGS. 5D and 5E are provided because of the scanning direction of the heads offset in the clockwise direction.
Thus, it will be understood that the errors wh and ws of the movement position of the heads are inevitably caused by the difference between the head heights on recording and reproducing and the difference between the inclinations of the axis of the rotary drum on recording and reproducing.
The variation in the tape velocity and the offset position of the heads relative to the tracks causes the differential voltages Va.sub.1, Va.sub.2, Vb.sub.1 and Vb.sub.2 of the TE voltage signals s.sub.5 output from the SH circuit 32 to move along the variation locus of FIGS. 5D and 5E while the errors wh and ws are maintained. For example, if the relationship of the tape velocity Vr on recording and the tape velocity Vp on reproducing is (Vr&lt;Vp), then they move in the positive direction. If it is (Vr&gt;Vp), then they move in the negative direction.
As noted from the foregoing, since the ATF control is accomplished by controlling the tape velocity v so that the average level of the TE voltage signals s.sub.5 is zero, the levels of the respective differential voltages are stable at the positions of FIGS. 5D and 5E.
One of the disadvantages of the prior ATF control system is that it tends to deteriorate the reproduction condition. More particularly, the heads ha and hb scan the corresponding tracks by ATF control but, if there is a difference between the inclinations of the rotary drum on recording and reproducing, then the reproduction will be made on the condition that the scanning locus is not parallel to the track direction and if the heads ha and hb scan at different height on recording and on reproducing, then the reproduction will be made on the condition that the center of the track is inconsistent with the center of the heads. This causes the reproduction condition to deteriorate. Since the prior ATF control system cannot detect the height error of the heads and the inclination error of the rotary drum, whether the reproduction condition is allowable cannot be determined.
Also, in the prior ATF control system, the TE voltage signals s.sub.5 output from the SH circuit 32 are renewed by the sequential differential voltages Va.sub.1, Va.sub.2, Vb.sub.1, Vb.sub.2 of FIGS. 5A through 5E every time the respective heads ha and hb scan the areas E.sub.1 and E.sub.2. Thus, if there is a head height error and an inclination error of the rotary drum on recording and on reproducing, then there is a large variation in the differential voltage levels in proportion to the magnitude of the errors. The level variation can be normally expressed by the TE voltage signals s.sub.5.
Thus, to provide for ATF control the average voltage signal s.sub.12 having the level variation components removed from the TE voltage signal s.sub.5, which variation components cause the errors of height and inclination. This average voltage signal can be obtained by a smoothing circuit, but such a smoothing circuit causes delayed response to information of movement of the head positions on the variation in the tape velocity.